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Regular

The plural form of most nouns is formed by adding s to the end of the word.

There are twelve boys and thirteen girls in that class.

When a word ends in sh, ch, s, z, or x, the plural is usually formed by adding es to the end.

We need three batches of cookies for three different classes at school.

When a word ends in [consonant] + y, the plural is formed by changing the y to ie and adding s.

I may live in only one country at a time, but I feel like a citizen of many countries.

In compound nouns, the principal word is the one made plural.

daughters-in-law, governors general, passers-by, ladies in waiting

Irregular

Many nouns referring to animals have the same form in the singular and in the plural.

The land sustained an abundance of deer and moose, and could also be used to raise sheep.

If a noun ends in f or fe the plural is usually formed by adding s, but is sometimes formed by changing the f or fe to a ve and adding s.

His beliefs told him that those loaves required sharp knives.

If a noun ends in o, the plural is usually formed by adding s, but is sometimes formed by adding es.

My heroes all play banjos.

Words borrowed into English from other languages sometimes follow the rules for pluralisation in English and sometimes those for pluralisation in the original language.

phenomenon, phenomena and thesis, theses from Greek; alumnus, alumni and alumna, alumnae from Latin; tableau, tableaux and corps, corps from French

Special Cases

Plurals of symbols, numbers (including years), and uppercase letters are usually formed by adding s.

He was concerned with the use of @s in formal writing since the 1990s.
I could not suppress a smile when I saw so many As on my transcript.

Plurals of lowercase letters are usually formed by adding ’s after the letter.

He wondered how many c’s and m’s were in the word “accommodate.”

For most proper names, the plural is formed simply by adding s to the end of the name, though when a proper name ends in s the plural is formed by adding es.

The three Jacks joined the three Jills to fetch three pails of water.
We always worked very hard to keep up with the Joneses.

Collective nouns (referring to groups of people, animals, or things) are usually treated as singular. If, however, you want to lay stress on the individual members rather than on the overall unit, you may treat the noun as plural:

A flock of birds is flying in a V formation, but A flock of birds are threatening our crops.
My family is strange, but My family are unpredictable in their tastes.